1. Field of the Invention
The present invention relates to a system for transmitting information to a moving object, and more particularly, to a system for transmitting to a moving object information of, for example, positional data characteristic of each given position on the ground using a light beam.
2. Description of the Related Art
Recently, there is a growing tendency toward developing what is known as a car navigation system which automatically guides a car to a destination, or indicates to a driver the present position of the car or the direction to be taken. Roughly speaking, the car navigation system can be divided into two classes. The first one is a system similar to that referred to as a self-contained navigation system in the aircraft navigation, and the second one, which is also used in the aircraft navigation, is a system similar to that utilizing the navigation aid installation.
The self-contained navigation system is a system in which the present position is evaluated by monitoring the moving speed and direction all the time and integrating them. In the system utilizing the navigation aid installation, for example, one receives radio waves from a plurality of radars provided on the ground, and then calculates one's own position taking into consideration the positions of the aid installations and the condition of the received radio waves. In another system, one knows one's own positional information by telecommunication. In the car navigation system, the system by the navigation aid installations is employed, in many case, for correcting the positional information obtained by the self-contained navigation system.
The self-contained navigation system in the car navigation includes the following. In the first one, the moving speed of a car is evaluated by averaging the moving speeds of the right and left rear wheels of the car, and the change of the travel direction is evaluated by the difference of the travel distances of the right and left rear wheels for a unit time. The coordinates of the present position of the car with the origin at a predetermined position can be known by summing up the above-described values by a computer.
In the second system of the self-contained navigation, the travel distance of a car is evaluated in the same manner as that of the first system, and the travel direction of a car is measured by a magnetic compass mounted on the car. Furthermore, as the third system, it is possible to measure the travel direction of a car with a gyro scope mounted on the car. In this case, the travel distance of a car also can be obtained by summing up the speed.
The positional data evaluated by the above-described self-contained navigation system can be used in a system for automatically guiding a car to a destination. However, at the present time, the automatic guiding technique as described above has not been developed yet. The technique which has been put to practical use to some extent includes one in which the positional data evaluated by the above-described self-contained navigation system is displayed on a CRT (Cathode-Ray Tube).
Some types of methods of displaying positional information on a CRT are proposed. These proposed methods include a method of displaying a locus, the present position and the travel direction of a moving car on a CRT, a method of displaying the locus of a moving car on a CRT with a displayed map for its background, and a method in which a road map is displayed on a CRT using coordinate data concerning the road, and the locus of a moving car is corrected with the road coordinate data, thereby displaying the present position of a car which always corresponds to the road.
Assuming that all the devices operate without any error, the above-described self-contained navigation system can be an ideal system. However, it is impossible in practice. For example, it is impossible to measure the travel direction of a car without error, and it can not be avoided that the speed measurement includes some error. Accordingly, the positional data obtained by the self-contained navigation system must be corrected by some means. The navigation aid installations are used in such a case.
FIG. 1 is a schematic diagram showing a navigation aid system currently proposed. Referring to FIG. 1, beacon providing facilities A1, A2, A3 and A4 are provided at intersections a1, a2, a3 and a4 of a road 130, respectively. Each of the beacon providing facilities emits such a weak radio wave that it covers only the range shown in a circle at each of the intersections. The radio waves include information indicating positions of the respective intersections. Thus, a car 132, for example, can know its own position by receiving the radio wave from the beacon providing facility A3 when it enters the intersection a3.
By correcting the positional data by the self-contained navigation system using the above-described beacon providing facility, the positional data can be kept precise.
However, it is pointed out that the system for transmitting to a car its own positional information by the above-described beacon providing facility has problems as described below. First, it is pointed out that the information to be transmitted to a car easily picks up radio noise. Especially, in a city, for example, where a great number of cars run and repeatedly start and stop, the radio noise produced by the cars themselves is very large. A great amount of radio waves is flitting in the air these days as can be seen from the spread of car telephone. Besides, the respective beacons must be very weak so as not to "interfere" with adjacent beacons. Accordingly, the signals transmitted to a car by the respective beacons can easily pick up the radio noise.
Furthermore, there is a geographical limitation in positioning a beacon providing facility for avoiding unfavorable effect among adjacent beacons as described above. According to the conventional method, it is impossible to provide a large number of beacon providing facilities in a geographically complicated place, and only rough positional information can be transmitted.